Compression tester

Stephen Newell · Jun 12, 2019

I have a front wheel drive oldsmobile I've been trying to do a compression test on. I had one tester which broke on me before I could complete the task. Today I bought a new one and read the instructions on it and it said to run the car until it get to it's running temperature, pull all the plugs and preform the test. Now with this car there is so little room it takes all day to do six cylinders and I can't picture trying to do it on a hot engine. To me it seems completely impossible to hook up the tester on half the cylinders hot without completely burning all the skin off your hands. Besides that two of them takes hours each to connect on a cold engine. I believe the engine would cool off before the tester could possibly be connected. There is no way to use some kind of heat resistant gloves. Any idea how I could preform the test.

Background on the vehicle it's a 87 Delta 88 which the timing gear went bad. That has been fixed but I believe either some of the valves have been bent or perhaps stuck. I'm getting a putt putt noise at the exhaust so I held a piece of paper over the exhaust pipe and it sucks the paper in and out fluttering. I'm just trying to make sure a valve job is really necessary. The car had sat for three years and I may try to use some penetrating oil on the stuck valves and see if that works first.

JD Seller · Jun 12, 2019

Take a inferred thermometer and start the car cold. Then shot the manifolds at each cylinder. Any that are not running/firing correctly will be colder. You need to do this fairly quickly as the warmth will spread fast with the car running. Then after you "find" the cold cylinders than do a compression test on just those cylinders.

I have never done a compression with a hot/warm motor being required. A motor that has been recently ran, would usually test correctly hot or cold. I can actually make the argument that hot could lead to false positive tests. I have had motors that had stuck rings, that will get better when the motor warms up. So hot they would have good compression but not at cold startup.

David G · Jun 12, 2019

I would not worry about hot engine, never have done it.

jake2 · Jun 13, 2019

You might want to be careful pulling plugs on a hot engine if the head is aluminum.

You might just pull the thread with the plug.

Married2Allis · Jun 13, 2019

You could check them all cold first. Then check the ones you can reach easliy with the engine hot. This should give you an idea of the differences for the ones you cannot reach, to identify the low cylinders.

Greg_Ky · Jun 13, 2019

Run a leak down test.

Mutt and Jeff · Jun 13, 2019

disable the fuel or spark
crank the engine
rr-rr-wheeee-rr-rr-rr

hey guess what #3 has no compression

mechanics use similar process while observing the cranking amp draw

Stephen Newell · Jun 13, 2019

It's a cast iron engine and the plugs have recently been removed and all have had anit-seize compound put on them.

timcasbolt · Jun 13, 2019

(quoted from post at 11:49:00 06/13/19) Run a leak down test.

How would that be easier than a compression test?

Stephen Newell · Jun 13, 2019

That is a relief. Not only do I have to reach in around the exhaust manifold to get to the plugs I literally have to lay on top of the motor to do it. It isn't possible to even see the spark plugs.

I've never done a compression test on a hot engine and thought I was doing something wrong. I've also never removed all the plugs at once. I've just done the test one at a time to keep from having to tag the wires. I can see how that would make the engine turn easier. In any case I should be able to locate the leaking valves.

Hobo,NC · Jun 13, 2019

Whut Mutt said...

I take it its skipping Those will sometimes bend a valve when it jumps time... If it did jump time the oil pan needs to come off and be cleaned out...

You can cut 6 links of rubber vacuum tubing about 1 1/2" long pull one plug wire at a time at the coil insert the tube in the coil tower and put the spark plug wire end over the tube. You can do one at a time are all 6 use a 12V test light are a ground wire to ground a spark plug wire out to locate the cylinder that's skipping by grounding the vacuum tube. The carbon in the vacuum tube will conduct electricity...

BTW It does not have to be hot to do a compression test....

Greg_Ky · Jun 13, 2019

Not any easier, will pin point the problem though. Rings, head
gasket, intake valve, or exhaust valve.

Steve@Advance · Jun 13, 2019

Checking compression hot is not that necessary.

It will still give you the info you need.

As long as the engine has been run recently, it will be close enough.

I'm surprised there was no mention of removing all the other plugs, having a fully charged battery, and propping the throttle plate open, and doing a dry/wet test. Those are more important than temperature.

A dry/wet test involves checking the compression first, then adding about a teaspoon of oil to each cylinder, spin out the excess, then run the test again. If little improvement, the rings are good. Major improvement means the rings are leaking.

Stephen Newell · Jun 13, 2019

I did the compression test this morning cold. It turned out the #1 cylinder was the culprit. It showed 60 psi where the rest showed between 105 and 140. Now I have to see if I can fix it without a valve job. It wasn't doing that three years ago when parked.

Stephen Newell · Jun 13, 2019

The battery is charged. I did remove all the plugs but I didn't open the throttle.

The car has a little over 100000 miles on it. I'm sure the rings are leaking a little but except for the rough idle the car runs really good. I think if I could get that one cylinder working like the rest I would be alright. Right now I can't imagine pulling the motor for an overhaul. Doing a valve job would be bad enough.

Stephen Newell · Jun 13, 2019

Not sure I can get the oil pan off. When I did the timing chain I tried to remove it and there is something overlapping one end of it where it wouldn't come off.

DR. EVIL · Jun 13, 2019

Thing I read written by Smokey Yunick decades ago in Popular Mechanics magazine was to loosen all the spark plugs a half to whole turn, put plug wires back on, restart engine and blip the throttle several times, rev it up good, any carbon particles you broke loose from the bottom of the plug your trying to blow out the exhaust port or chew up into tiny bits so the exhaust valve seats properly again. Then shut off engine and remove plugs and test compression. A false low cylinder happens more than you would think.

Steve@Advance · Jun 13, 2019

How much has it been run since sitting for 3 years?

It might help to take it for a run, get some oil circulating, some rust off the valves and seats, some fresh fuel flowing through the system, then try it again.

Most V6 engines don't fire even, so the exhaust may sound a little off even when it's running right.

But 60 is too low...

Stephen Newell · Jun 13, 2019

It's been driven less than ten miles since it has sat. It's not licensed or insured so I don't go far with it. I live near a highway and got it up to 55 for about a mile once.

Over the course of working on it I removed the old fuel even though it hadn't gone actually bad yet and replaced it with a higher octane. I also removed all the fuel injectors and cleaned them.

The putt putt noise coming from the exhaust is something new though. It also idles rough and I have to hold down on the accelerator for a little while when I first start it or it will die.

Stephen Newell · Jun 13, 2019

I don't have the tools for a leak down test anyway. I'm pretty certain If I can't free up the valves on that one cylinder I will end up pulling both heads and have a valve job done.

Jack345 · Jun 13, 2019

Sea Foam time! For less than $10 I've had good luck with making engine run better... improve compression.

guido · Jun 13, 2019

Hello Stephen Newell,

Squirt a little oil in the cylinder, chances are the top ring may be cold stuck. Then check the compression, bet you will be more then 60. Run it for awhile as suggested, it may clear up, but do the oil squirt first,

Guido.

Stephen Newell · Jun 13, 2019

How is that used?

Jack345 · Jun 13, 2019

Best to read can, going from memory get engine up to operating temp, get Seafoam into take system, via large vacuum line or carburetor .Better yet check out a YOUTUBE video

https://www.youtube.com/watch?v=u6UeJXkzDW8

Stephen Newell · Jun 14, 2019

I would probably have to use the air intake like in the first video as this car is fuel injected.

The problem is definitely with the exhaust valve. I brought the piston well up into it's compression stroke and hooked the cylinder up to compressed air. The air was blowing straight out the exhaust.

Jack345 · Jun 14, 2019

Yeah, injected models are a little hard to access.. I did note that in one of the videos Seafoam has a hose the reaches into intake from can, Let us know if it helped.

Stephen Newell · Jun 14, 2019

I picked up a can of the seafoam today and will give it a try tomorrow.

Stephen Newell · Jun 15, 2019

Overall the seafoam seem to help the car but it didn't help that one cylinder. It's still running at 60 psi.

Jack345 · Jun 16, 2019

It may come back with using it.....if it was not that way when parked a few years ago. Did you squirt some oil into cylinder to see if rings or valves ? If rings compression test would read higher oil makes little difference to valves while doing compression check

Stephen Newell · Jun 16, 2019

I started by trying to put compressed air in the cylinder and it leaked straight out the exhaust so I didn't try the oil. I did look in the cylinder with an inspection camera and the cylinder walls looked fine. The top of the piston was really cruddy though. I might attach the compressor again and whack the exhaust valve a few times and see if it blows some obstruction through. I'm about to the point where I'm going to pull the heads. I hate to, the car runs really good except for the idle but if I don't I'll end up screwing up the piston too and that would be a bigger job. I just shouldn't have let it sit that long but it kept giving me the impression the problem was with the electronics and I couldn't find the problem. I'd change something and it would start and run and then the next day quit. I think I've changed the ignition module four times. I'd work on the car for a few weeks and give up. Then someone put me onto the timing chain so I replaced it. The old one the teeth were stripped clean on one side. I'm not the only one that couldn't figure that car out. When my mother had it she had probably a dozen different mechanics including the dealer where the car was purchased work on it and couldn't make it dependable. The timing chain had to have been bad then as the car has only been driven about 800 miles since I've had it. Fortunately the first 600 miles of that 800 miles it did fine as I drove it across four states to get it home.

Jack345 · Jun 16, 2019

If you were @ TDC on power stroke & hear air going past exhaust valve ..thats the problem! & will need a valve job.

Stephen Newell · Jun 17, 2019

That is the conclusion I got. Too bad I hadn't come to that conclusion while I had it torn down for the timing chain. At least I know what the car needs and can just do it. It's been very frustrating tinkering with it over the last three years trying to figure out what was the problem. Someone at an Oldsmobile forum said I should be getting 150 psi and I was only coming close on one cylinder so all of them need some help.

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